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No-till tool for clay soil: daikon radishes

In 2001, University of Maryland’s Steve Groff and Dr. Ray Weil started developing daikon radish (Raphanus sativus L. var. niger J. Kern.) as a cover crop to improve soil health, break up soil hardpans and control weeds. Although daikon radish — also known as Tillage Radish® (trademarked by Cover Crop Solutions), forage radish or Japanese radish — has many benefits, it’s their tilling and breaking soil hardpans ability that sparked interest on a recent #groundchat.

The daikon radish’s “super carrot” taproot drills down two to four feet into the soil with a pressure of 290 psi, forming channels in the soil after they desiccate and decay over winter. It’s these channels that reduce compaction and improve soil tilth, which improves water infiltration and surface drainage. The channels also allow the soil to warm up quicker in the spring!

“It is the fine secondary roots that do the most good,” [in breaking soil hardpans], says Kevin Elmy of Elmy’s Friendly Acres Seed Farm. When the taproots hit the hardpan, fine roots are sent out to find a crack in the hardpan. Eventually the roots crack open the hardpan.

Even this “small” root created a pore through the soil to aerate, recover nutrients, energized the soil. Doing its job. It is the fine secondary roots that do the most good. Photo courtesy of Kevin Elmy http://www.friendlyacres.sk.ca

But there are some growers/farmers that haven’t had success with tilling radish.

Dr. Matt Ruark, soil scientist at University Wisconsin-Extension, agrees with the tilling aspect of daikon radish, but in his research trials, the daikon radish roots didn’t penetrate and break through the hardpan. It must be mighty hard hardpan in Wisconsin because some farmers have had success breaking through their hardpan.

I emailed Kevin Elmy about rumblings and grumblings on the lack of hardpan breakage with daikon radish. He quickly emailed me back:

“In most cases, they have seeded into dry soil, poor seedbed preparation, lack of nutrients, all resulting in nothing growing. From our research, we have shown the roots will grow from 2-3 cm down per day of growth.”

“I have had an Ontario producer saying they did not work on corn silage ground. After asking questions and looking at pictures it did not make sense until I asked about manure. He forgot to tell me that 25,000 gallons of hog manure was added last fall, plus a “normal” amount of fertilizer. The tubers were growing so fast, they could not grow into the ground faster than their growth rate, resulting in a tuber sticking out of the ground about 18” with poor root growth. Plus the seeding rate was too low, resulting in larger tubers.”

For tillage purposes daikon radishes are best planted in late summer. Timing is important. Grow daikon radishes 40 to 60 days before a killing frost, which is three consecutive nights at -8 C. This enough time for roots to grow which allows enough time for the radishes to grow and do their thing. For Northern gardeners, this usually means a late August seeding!

“If the goal is to break up hardpan with a pure stand of tillage radish, researchers recommend using seven to 10 pounds of seed per acre,” says Elmy in his 2012 Grain Newsarticle, Three uses for forage radish.

Planting daikon radishes in the spring poses problems. It seems that spring planted daikon radishes bolt quickly and grow much less root and shoot than fall plantings. Defeats the purpose of growing long roots to break up the hardpan.

Daikon radishes don’t grow well in low nitrogen soil. There are heavy nitrogen feeders. Daikon radishes grown in nitrogen-deficient soil won’t grow through compacted soil or compete very well with weeds.

They also won’t grow very well in wet soil. Avoid planting on spots that collect standing water or tend to stay wet for a long time.

Of course, gardeners shouldn’t plant daikon radishes in the same spot every year. It will lead to a build up of pests and diseases that feed on Brassicaceae family. And definitely don’t plant any vegetables from Brassicaceae family the next year.

It appears that daikon radishes do work, but you need to sow the seeds at the right time and give them the proper growing conditions. But if you have a super hardpan, well you could be between a rock and a hard place.

Cristina is a garden writer and instructor who specializes in the importance of soil in gardening.

Cristina has been instructing adults for over 30 years. Cristina also writes Soil Groundwork blog and runs a tweet chat called #groundchat, which discusses different soil-related topics on Twitter every Friday at 2 pm EST.

She is a longtime gardener (30+ years) with two degrees in horticulture. Her Masters covered aspects of soil and plant nutrition. Cristina has gardened in many types of soils. From sandy soil at 10 years old to clay soil garden in southern Ontario.

How interesting. I grow things in raised beds but under the “raised” part of the bed is heavy clay. So far I have just added more humus every year and improve the soil by going up. I should try “going down” with daikon.
Thank you for this very informative post.

I was just writing a post for my blog and googled the spelling of daikon radish. This was just the subject I was remembering from the Master Gardener classes. The instructor explained about using the radish to breakup the hardpan. But he had said they need to be left in the soil for it to work? Is this true? He said you’d have to worry about the smell of the rotting radish. Just wanted your opinion. ( My blog is about NOT using something in a hypertufa trough with such a long taproot. Trying to make a funny. ) If not, I may be sowing a lot of radish seed this August here in SW Ohio!

The instructor is right, Kim. You do need to leave the daikon radish in the soil to work its hardpan magic. And yes,it does smell when they rot. Daikon radishes do belong to the Brassicaceae family after all 😉 Think of boiling cabbage, Brussels sprouts.

Very informative. I’ve converted to no/low tillage in my veggie garden, as per Eliot Coleman’s practices, and beginning the process of enhancing the soil using biochar (biologically activated charcoal). Biochar and tillage radish seem like a great combination to open the soil and allow the migration of the biochar into the clay. The Amazonian Indians effectively used biochar on what are known to be some of the poorest soils in the world, the Amazon Rain Forest. After 500 years of no cultivation, the charcoal amended soil is still rich and productive. The process also sequesters huge amounts of carbon dioxide – for centuries! Interesting YouTube video on the discovery of biochar at https://www.youtube.com/watch?v=0Os-ujelkgw If you’re interested, just Google biochar.

Fantastic work, Garth! I am a great believer in the no/low tillage. Just makes sense! Last year on #groundchat we had Thea Whitman explain the ins and outs of biochar on #groundchat. Thea is doing her PhD on biochar at Cornell University. Here is Thea’s blog http://theawhitman.wordpress.com

I still waiting to see how that old technology will translate into gardening terms. It still has some issues to sort out i.e. burning wood (puts more pollutants into the air). I also curious to see if “modern” scientists are going to use the fish and biochar together. I read the Amazonian Indians didn’t use biochar alone. But I am excited by biochar’s potential. Only time will tell.